The market for retreaded pneumatic tires is almost entirely in the arena of trucking, aircraft, and earthmoving vehicles. In the trucking market, the carcass of a tire is expected to last several hundred thousand miles, and be amenable to having a new tread adhered to it several times. New truck tires are quite expensive, and are therefore bought with the expectation that their high initial costs are offset by the long service life of the carcass, and the low comparative cost of retreading.
A variety of procedures and different types of equipment are available for use in recapping or retreading pneumatic tires. One of the first steps in retreading a worn tire is to remove remaining tread material from the tire carcass, for example, by a sanding procedure known as buffing. Next a layer of green (uncured) rubber, known as “cushion gum,” is applied to the carcass. This layer of extruded uncured rubber may be stitched or adhesively bonded to the carcass. Next, a tread layer is applied atop the layer of cushion gum. In the cold recapping process, the tread is cured rubber, and has a tread pattern already impressed in its outer surface. The tire is then placed in an autoclave, and heated under pressure for an appropriate time to induce curing of the gum layer, and binding of the gum layer to the tread and the carcass. In the hot recapping process, the tread is uncured rubber, and has no tread pattern. The tire is then placed in a tire mold and heated under pressure for an appropriate time to cure the gum layer and the tread, and to cause the gum layer to bind with the tread and the carcass. [The term “cure” refers to the formation of cross-links between the elastomer molecules in the rubber compound, otherwise known as vulcanization].
However, in recapping procedures it is often necessary to employ rubber compounds with short vulcanization times. A high cure rate necessitates the use of ultra-accelerators in the rubber compound, which creates challenges for the employment of the compound. The accelerators, at ambient temperature, may begin to react with other components of the compound. They must therefore be stored under refrigeration to prevent premature curing of the compound or evolution of the cure rate. More importantly, the cure rate (also referred to as “cure law”) may increase upon green aging of the compound, leading to the need for a process modification or possibly hindering performance of the product.
Various methods have been proposed to prevent premature curing of the cushion rubber compound. These include Japanese Patent No. 48052678 (1973); Japanese Patent No. 48052679 (1973); German Patent Application No. DE19754341 (1999); International Patent No. WO99/27012 (1999); European Patent Application No. EP1034216 (2000); Japanese Patent No. 59022942 (1984); Japanese Patent No. 52069455 (1977). Certain other references have addressed encapsulation of materials: U.S. Pat. No. 4,092,285 (1978); U.S. Pat. No. 6,058,994 (2000); U.S. Pat. No. 4,461,854 (1984). None of these methods has proven entirely satisfactory.
Therefore, it would be beneficial to have a rubber composition which did not require refrigeration, and which limited its cure rate change through its useful life, or, better still, which showed no cure rate change through its useful life.